End-to-end connections of a data transmission system, such as a telephone network, often show long transit time delays, in consequence of which echo is observed, for instance, in the case of normal speech, when a signal is reflected from the far end of a connection back to the talker.
An echo canceller is a device for processing a signal, such as a speech signal, so as to reduce echo by subtracting estimated echo from the echo (signal) occurring in a connection. The echo canceller can be either digital or analog. Echo cancelling devices are at present realized by digital signal processing, by means of which it is possible to model echo paths including considerably long transit time delays. A digital adaptive filter is then used, in which the coefficients of the filter modelling the echo path are updated on the basis of a correlation of the speech signal and the returning echo signal.
Since the echo path in principle is different of each call, it is necessary to apply to the echo canceller a method which always adapts itself to a new echo path at the start of a call. It takes a little time to converge the echo canceller, i.e. to create a model of the echo path after the speech has begun in the connection, and therefore, echo occurs momentarily at the start of the call before the echo canceller has been converged.